While a pneumatic tire is being vulcanized, if a trapped air is generated between a raw tire and a molding surface of a mold in a process of pressing the raw tire against the molding surface of the mold by swelling the raw tire from its inside by use of a bladder, the trapped air portion may cause a defective appearance that a concave is generated in the tire surface. The defective appearance is called light failure. Particularly a pneumatic tire whose sidewall surface is embossed with marks such as large-sized letters and patterns has a problem that, if a rigid member in the tire, such as an end portion of a bead filler or a folded-back end portion of a carcass layer, laps over an embossed mark in such a way to cut across the embossed mark, a light failure is more likely to occur in the embossed mark due to the difference in rigidity between the embossed mark and the rigid member.
As a conventional remedy for the light failure, a number of vent holes are provided in a molding surface of a vulcanization mold to discharge air to the outside of the mold, thereby preventing a trapped air from being generated. However, unvulcanized rubber flows into the vent holes after discharging air, and thus forms vent spews shaped like beards. Removal of the vent spews after vulcanization requires long man-hours. In addition, it is difficult to remove the vent spews, in the removal work, so as to have a level surface of the tire, because the vent spews are soft. Since parts of the vent spews thus remain as protrusions after the removal, there arises a problem that the remaining parts of the vent spews spoil the appearance of the tire.
With this taken into consideration, Patent Document 1 has proposed to reduce the number of vent holes provided in each sidewall molding surface in the following manner. Vent grooves are formed in the sidewall molding surface at predetermined intervals in the tire circumferential direction in such a way as to cut across the sidewall molding surface in a radial direction, and one vent hole is provided to each radial vent groove. However, because these vent grooves are each formed in the tire radial direction, it is impossible to fully prevent light failures if intervals at which the vent grooves are arranged in the tire circumferential direction are not set narrower. This restricts the reduction of vent holes in number.    Patent Document 1: Japanese patent application Kokai publication No. 2004-136617